The present invention relates generally to the field of electromechanical pressure sensors and more particularly to the field of ceramic capacitive pressure transducers.
Ceramic capacitive pressure transducers are known and generally comprise parallel plate capacitor electrodes separated by an air gap wherein the spacing between the parallel plate electrodes is altered in response to a sensed pressure thereby changing the capacitance created by these electrodes. Generally, one capacitor electrode plate is deposited on a top end surface of a relatively thick cylindrically shaped ceramic base substrate while the other capacitor electrode is deposited on a relatively thin discshaped ceramic pressure sensing diaphragm. An annular glass insulating ring is deposited on the peripheral portion of the base substrate top surface and is used to bond the diaphragm to the base substrate as well as to space the diaphragm electrode a predetermined distance away from the base substrate electrode. Typically, the diaphragm, the annular glass ring and the base substrate are assembled into a sandwich type structure and then heated to form an integral assembly such that the capacitor electrodes are spaced apart by a predetermined distance totally dependent upon the thickness of the annular glass ring. Generally, a vacuum entryway hole is provided though the base substrate, and through this hole a predetermined reference vacuum pressure is applied to an air cavity formed by the diaphragm, the annular glass ring and the base substrate. Subsequently, the vacuum entryway is sealed so that the internal cavity will maintain (store) a predetermined reference vacuum pressure. By applying various degrees of pressure to the exterior of the capacitive pressure transducer, the transducer diaphragm is flexed by predetermined amount and this results in changing the capacitance created by the capacitor electrodes since the flexing of the diaphragm changes the spacing between the electrodes. Thus by monitoring the capacitance created by the electrodes, the ceramic capacitor transducer will produce an electrical signal related to the magnitude of the exterior pressure applied to the diaphragm as compared to the magnitude of the reference vacuum pressure. Such transducers are readily adaptable for sensing vacuum pressures generated by automobile internal combustion engines.
Typically, the nominal distance between the base and diaphragm capacitor electrodes is very small so that small changes in exterior pressure will result in relatively large changes in the capacitance created by these electrodes. In prior art ceramic capacitor transducers of this type, the top surface of the base substrate, as well as the diaphragm surface on which the diaphragm electrode is deposited, are substantially planar. Because of this and because the nominal distance between the electrodes has to be kept relatively small, the volume of the internal cavity which stores the reference vacuum pressure is very small. This results in these prior art capacitive transducers having relatively short life times whenever any appreciable leakage rates for the internal cavity exist. Also, since previous ceramic capacitive transducers have base substrates which are substantially solid, except for a narrow vacuum entryway hole, the base capacitor electrodes is surrounded with ceramic material having a high dielectric constant, and the end result is that the capacitor is much more susceptible to capacitive fringing effects. In addition, prior art capacitive transducers encounter capacitive fringing problems due to the effect of conductive surfaces or particles which are located exterior to the internal cavity but close to the thin flexible diaphragm. These conductive surfaces or particles create variable additional coupling between the capacitor electrodes and therefore tend to make these capacitive pressure sensors unreliable in that a fixed value of capacitance would not always be generated in response to a fixed value of external pressure being applied to the diaphragm.